The present invention relates to a reactor tube for thermal cracking or reforming hydrocarbons, in particular a reactor tube which prevents deposition and building up of solid carbon thereon accompanied by chemical reaction of hydrocarbon and further prevents carburization.
Reactor for thermal cracking and reforming of hydrocarbon used here is in tubular form and passes hydrocarbon therethrough in liquid or gaseous form under high pressures and temperatures for thermal cracking or reforming, in the presence or absence of a catalyst layer. The material heretofore used for such reactors is Fe-CR-Ni austenitic heat resisting steel which contains large amounts of Ni and Cr and has been generally applied for equipments at high-temperature use. It has been the usual practice to increase the Ni content to enhance heat resisting property of tube material to be used at higher temperatures.
Since thermal cracking or reforming of hydrocarbons accompanies deposition of solid carbon, when reaction is continued by using such reactor tube made of Fe-Cr-Ni steel as aforesaid, solid carbon inevitably deposits and builds up on the wall surface (inner wall surface, outer wall surface or both inner and outer wall surfaces depending on the way of use of reactor tube) to be in contact with hydrocarbons. When such deposit of solid carbon is left unattended, it not only obstructs the passage of fluid containing hydrocarbon through the tube but also remarkably reduces overall heat transfer coefficient in the reaction heat supplied or removed from outside the tube and thus operation becomes difficult to be continued. As the result periodical shutdown of operation is required for removal of carbon deposits by various methods, so-called decoking, although the reactor is to be operated continuously as a rule. Besides, the conventional reactor tube as aforesaid presents such problems as deterioration of tube material due to carburizing through reaction wall surface, particularly a remarkable reduction of ductility and danger of generation of cracks due to embrittling of tube material under high pressures.
In order to solve the above problems, we have carried out intensive research and found out that the reason for the remarkable carbon deposits in the reactor tube made of Fe-Cr-Ni heat resisting steel is that Ni contained in the steel material acts catalytically to accelerate deposition of solid carbon on the tube surface through hydrocarbon and that there exists a correlation between the amount of solid carbon deposits and Ni content in the tube material and by restricting such Ni content, it is possible to inhibit and prevent deposition of solid carbon on the tube surface. As for carburization, when steel material of the tube contains appropriate amount of Mn and Nb, carburization from the tube wall surface is effectively restricted and deterioration of tube material can be prevented.